Systemic acquired resistance (SAR) is a state of heightened defense which is induced throughout a plant by an initial infection; it provides long-lasting, broad-spectrum resistance to subsequent pathogen challenge. Recendy we identified a phloem-mobile signal for SAR which has been elusive for almost 30 years. It is methyl salicylate (MeSA), an inactive derivative of the defense hormone, salicylic acid (SA). This discovery resulted from extensive characterization of SA-binding protein 2 (SABP2), a protein whose high affinity for SA and extremely low abundance suggested that it might be the SA receptor. Instead we discovered that SABP2 is a MeSA esterase whose function is to convert biologically inactive MeSA in the systemic tissue to active SA. The accumulated SA then activates or primes defenses leading to SAR. SABP2's esterase activity is inhibited in the initially/primary infected tissue by SA binding in its active site; this facilitates accumulation of MeSA, which is then translocated through the phloem to systemic tissue for perception and processing by SABP2 to SA. Thus, while SABP2 is not the SA receptor, it can be considered the receptor for the SAR signal. This study of SABPs not only illustrates the unexpected nature of scientific discoveries, but also underscores the need to use biochemical approaches in addition to genetics to address complex biological processes, such as disease resistance.

Soybean [Glycine max (L.) Merr.] is an important crop worldwide. It has been widely consumed for protein, oil and other soy products. To develop soybean cultivars with greater resistance against pests and improved meal quality, it is important to elucidate the molecular bases of these traits. This dissertation aims to investigate the biochemical and biological functions of soybean genes from four gene families, which are hypothesized to be associated with soybean defense against pests and soybean meal quality. There are three specific objectives in this dissertation. The first one is to determine the function of components in the salicylic acid (SA) signaling pathway in soybean resistance against soybean cyst nematode (Heterodera glycines, SCN). The second one is to determine whether insect herbivory induce the emission of volatiles from soybean, and if so, how these volatiles are biosynthesized. The third objective is to identify and characterize soybean mannanase genes that can be used for the improvement of soybean meal quality. The soybean genome has been fully sequenced, which provides opportunities for cross-species comparison of gene families of interest and identification of candidate genes in soybean. The cloned cDNAs of putative genes were expressed in Escherichia coli to produce recombinant enzymes. Through biochemical assays, these proteins were proved to be soybean salicylic acid methyltransferase (GmSAMT1), methyl salicylate esterase (GmSABP2-1), α[alpha]-farnesene synthase (GmTPS1) and E-β[beta]-caryophyllene synthase (GmTPS2), and endo-β[beta]-mannanase (GmMAN1). Through a transgenic hairy root system harboring overexpression of GmSAMT1 and GmSABP2-1, both of these two genes were evaluated for their biological function related to resistance against SCN. The results showed that the over-expression of GmSAMT1 and GmSABP2-1 in the susceptible soybean background lead to enhanced resistance against SCN. Among four putative soybean mannanase genes, one gene was cloned and characterized. GmMAN1 showed the endo-β[beta]-mannanase hydrolyse activity and can hydrolyze cell walls isolated from soybean seeds. In summary, using comparative and functional genomics, a number of genes involved in soybean defense and meal quality were isolated and characterized. This study provides novel knowledge and molecular tools for the genetic improvement of soybean for enhanced resistance and improved meal quality.